1. Field of the Invention
This invention relates to a method of and an apparatus for detecting the secondary distortion of optical type recorded information, that is, detecting the presence of secondary distortion contained in the information recorded on a recording medium such as an optical disc or a magneto-optical disc.
2. Related Background Art
When information is to be recorded in the form of a recording pit comprising a mechanical uneven configuration on an optical disc by the application of a laser beam based on recording information, or when information is to be recorded in the form of a recording bit by the vertical direction of magnetization on a magneto-optical disc, a binary signal whose duty ratio is 50% has heretofore been recorded to obtain a proper laser driving power, the laser power has been calibrated so that the duty ratio of the recorded information recorded on the medium is 50%, so that the recorded information contains no secondary distortion, and recording has been effected by the calibrated laser power.
Now, a spectrum analyzer is usually used for the measurement of secondary distortion contained in the information recorded on an optical disc. That is, the recorded information of the optical disc is optically read by a reading head and converted to an electrical signal, and the information reproducing signal obtained from the reading head is analyzed by the spectrum analyzer.
When the analysis of the recorded information has been effected by such spectrum analyzer, in the case of an ideal recording signal whose duty ratio is 50% the result of the measurement of the recorded information by the spectrum analyzer is such that, as shown in FIG. 12A of the accompanying drawings, spectral components of only odd orders such as 3fs, 5fs, . . . are obtained for the recording frequency (basic frequency) fs.
On the other hand, when the duty ratio of the recording signal is smaller or greater than 50%, the result of the measurement by the spectrum analyzer is as shown in FIG. 12B of the accompanying drawings, and in this case, spectral components of even orders such as 2fs, 3fs, 4fs, . . . also occur for the recording frequency fs, and this secondary spectral component which is 2fs is called secondary distortion.
However, in the conventional secondary distortion detecting method using a spectrum analyzer, it has been difficult, for the following reasons, to automatically control the laser power which provides a recording signal so that, secondary distortion may not occur, that is, so that the duty ratio of recorded information may be kept at 50%.
Firstly, in the result of the measurement by the spectrum analyzer, even if the duty ratio of the recording signal is smaller than 50% or conversely greater than 50% there appears a secondary spectral component representing the secondary distortion as shown in FIG. 12B, and the polarity thereof cannot be detected. Accordingly, even if the secondary distortion can be detected, in which direction the duty ratio of the recording signal deviates relative to the 50% duty ratio for which no secondary distortion occurs cannot be known and thus, judgment as to whether an increase or decrease in the laser power is required is not possible.
Secondly, when the recording frequency is low, that is, when the recording bit is large, there is the problem that the detection accuracy of the secondary spectral component by the spectrum analyzer is reduced, and this has led to the necessity of increasing the recording frequency fs.
However, if the recording frequency becomes high to a certain extent, it becomes impossible due to the MTF (modulation transfer function) frequency characteristic of the reading optical system to detect the secondary spectral component. MTF refers to the frequency characteristic of the sensitivity with which the optical system detects the recorded information, and it is the characteristic shown in FIG. 13 of the accompanying drawings. As is apparent from FIG. 13, if the recording frequency fs is made high to a certain extent as shown, MTF becomes zero at 2fs which provides the secondary spectral component, and the signal of 2fs cannot be detected.